A search for the $K^+\to\mu^-\nu e^+e^+$ decay

TL;DR

This paper reports a search for a rare kaon decay that violates lepton number or flavor conservation, setting a new upper limit on its branching fraction, significantly improving previous constraints.

Contribution

The study provides the most stringent upper limit to date on the $K^+ o u e^+ e^+$ decay branching fraction, using data from the NA62 experiment.

Findings

Upper limit of $8.1 imes 10^{-11}$ at 90% CL

Improved sensitivity by a factor of 250 over previous searches

Constraints on lepton number and flavor violation in kaon decays

Abstract

A search for the $K^+\to\mu^-\nu e^+e^+$ decay, forbidden within the Standard Model by either lepton number or lepton flavour conservation depending on the flavour of the emitted neutrino, has been performed using the dataset collected by the NA62 experiment at CERN in 2016--2018. An upper limit of $8.1\times 10^{-11}$ is obtained for the decay branching fraction at 90% CL, improving by a factor of 250 over the previous search.